1. Field of the Invention
The present invention relates to an improvement in an electronic sphygmomanometer.
2. Description of the Prior Art
An example of an electronic sphygmomanometer is disclosed in U.S. Pat. No. 4,273,136, "ELECTRONIC SPHYGMOMANOMETER", issued on June 16, 1981. Generally, in the electronic sphygmomanometer, the arm cuff pressure is first increased to a level higher than an assumed systolic pressure value. A microphone is disposed in the arm cuff to detect the Korotkoff sounds during the decrease of the cuff pressure. When the first Korotkoff sound is detected, the electronic sphygmomanometer memorizes the cuff pressure as the systolic pressure. When the Korotkoff sounds disappear, the system memorizes, as the diastolic pressure, the cuff pressure at a time when the last Korotkoff sound was detected.
That is, the diastolic pressure is determined when the Korotkoff sounds are not detected for a predetermined period of time. However, there is a possibility that the Korotkoff sounds are not detected due to the auscultatory gap even though the cuff pressure is above the diastolic pressure. In such a case an accurate measurement is not achieved.
In order to prevent the erroneous determination caused by the auscultatory gap, a time period, during which the disappearance of the Korotkoff sounds is expected, is selected at a considerably long time in the conventional system. This lengthens the measuring time of the electronic sphygmomanometer. The thus lengthened period will provide noises which are erroneously determined as the Korotkoff sounds. In another conventional system, the Korotkoff sound disappearance determination period is selected longer when the cuff pressure is above a predetermined value, and is selected shorter when the cuff pressure is below the predetermined value. This is based on the assumption that the auscultatory gap scarcely occurs near the diastolic pressure point. However, the assumption is not always correct. More specifically, a person who shows the auscultatory gap near the systolic pressure point shows the auscultatory gap near the diastolic pressure point with a considerably high probability. The above-mentioned first and second conventional systems therefore do not ensure an accurate blood pressure measuring.